The present invention will be described in the context of chrominance signal processing in a television receiver though it is not limited to this application. In a television receiver, received video signal is separated into luminance and chrominance components. These components are processed separately and then recombined to produce R, G and B signals to drive an image display device.
The chrominance component includes, in sequential format, a synchronizing color burst followed by chrominance image formation. The amplitude of the color burst and the ratio of the amplitude of the color burst to the amplitude of the image formation is generally fixed by convention. Not infrequently, the magnitude of the color burst (and the image information) of the received signal deviates from the desired level due to faulty broadcast equipment or the transmission medium etc. To compensate for these deviations and restore the chrominance signal to nominal levels, conventional receivers include automatic chrominance control (ACC) circuits. The ACC circuits compare the burst magnitude to a preset reference and amplify or attenuate the chrominance signal to maintain the burst signal amplitude constant at the desired level.
It happens, due to faulty ACC operation or differential color burst image information deviations, that the ACC circuit raises the chrominance signal magnitude undesirably high. The effect of this is to reproduce images with excessively bright colors. To compensate for this latter contingency, chroma overload circuitry is provided which monitors the chrominance signal downstream from the ACC circuitry, and attenuates the chrominance signal when its magnitude exceeds a predetermined amplitude. In conventional analog receivers, the chroma overload function may be implemented with a simple gain controlled amplifier to provide the signal attenuation and a diode and low pass filter to provide detection.
Now consider a receiver which processes video signals digitally, i.e. using binary arithmetic. In a digital receiver gain/attenuation implies multiplication, and binary multipliers are relatively large, expensive devices and are to be avoided. Secondly, there is no binary device which performs a nonlinear function corresponding to a diode detector. And finally, it may not be possible to access the chrominance signal at a point in the signal path where it is most appropriate to perform chroma overload protection.